class Keymap::ConnectionAdapters::ConnectionPool

Connection pool base class for managing connections.

Introduction

A connection pool synchronizes thread access to a limited number of connections. The basic idea is that each thread checks out a connection from the pool, uses that connection, and checks the connection back in. ConnectionPool is completely thread-safe, and will ensure that a connection cannot be used by two threads at the same time, as long as ConnectionPool’s contract is correctly followed. It will also handle cases in which there are more threads than connections: if all connections have been checked out, and a thread tries to checkout a connection anyway, then ConnectionPool will wait until some other thread has checked in a connection.

Obtaining (checking out) a connection

Connections can be obtained and used from a connection pool in several ways:

Simply use Keymap::Base#connection. Eventually, when you’re done with the connection(s) and wish it to be returned to the pool, you call Keymap::Base.clear_active_connections!.
Manually check out a connection from the pool with Keymap::Base.connection_pool.checkout. You are responsible for returning this connection to the pool when finished by calling Keymap::Base.connection_pool.checkin(connection).
Use Keymap::Base.connection_pool.#with_connection(&block), which obtains a connection, yields it as the sole argument to the block, and returns it to the pool after the block completes.

Connections in the pool are actually AbstractAdapter objects (or objects compatible with AbstractAdapter’s interface).

Options

There are two connection-pooling-related options that you can add to your database connection configuration:

pool: number indicating size of connection pool (default 5)
+checkout _timeout+: number of seconds to block and wait for a connection before giving up and raising a timeout error (default 5 seconds).

Attributes

automatic_reconnect[RW]

connections[R]

spec[R]

Public Class Methods

new(spec) click to toggle source

Creates a new ConnectionPool object. spec is a ConnectionSpecification object which describes database connection information (e.g. adapter, host name, username, password, etc), as well as the maximum size for this ConnectionPool.

The default ConnectionPool maximum size is 5.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 67
def initialize(spec)
  super()

  @spec = spec

  # The cache of reserved connections mapped to threads
  @reserved_connections = {}

  @queue = new_cond
  # 'wait_timeout', the backward-compatible key, conflicts with spec key 
  # used by mysql2 for something entirely different, checkout_timeout
  # preferred to avoid conflict and allow independent values. 
  @timeout = spec.config[:checkout_timeout] || spec.config[:wait_timeout] || 5

  # default max pool size to 5
  @size = (spec.config[:pool] && spec.config[:pool].to_i) || 5

  @connections = []
  @automatic_reconnect = true
end

Public Instance Methods

active_connection?() click to toggle source

Is there an open connection that is being used for the current thread?

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 100
def active_connection?
  synchronize do
    @reserved_connections.fetch(current_connection_id) {
      return false
    }.in_use?
  end
end

checkin(conn) click to toggle source

Check-in a database connection back into the pool, indicating that you no longer need this connection.

conn: an AbstractAdapter object, which was obtained by earlier by calling checkout on this pool.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 251
def checkin(conn)
  synchronize do
    conn.run_callbacks :checkin do
      conn.expire
      @queue.signal
    end

    release conn
  end
end

checkout() click to toggle source

Check-out a database connection from the pool, indicating that you want to use it. You should call checkin when you no longer need this.

This is done by either returning an existing connection, or by creating a new connection. If the maximum number of connections for this pool has already been reached, but the pool is empty (i.e. they’re all being used), then this method will wait until a thread has checked in a connection. The wait time is bounded however: if no connection can be checked out within the timeout specified for this pool, then a ConnectionTimeoutError exception will be raised.

Returns: an AbstractAdapter object.

Raises:

ConnectionTimeoutError: no connection can be obtained from the pool within the timeout period.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 204
def checkout
  synchronize do
    waited_time = 0

    loop do
      conn = @connections.find { |c| c.lease }

      unless conn
        if @connections.size < @size
          conn = checkout_new_connection
          conn.lease
        end
      end

      if conn
        checkout_and_verify conn
        return conn
      end

      if waited_time >= @timeout
        raise ConnectionTimeoutError, "could not obtain a database connection#{" within #@timeout seconds" if @timeout} (waited #{waited_time} seconds). The max pool size is currently #@size; consider increasing it."
      end

      # Sometimes our wait can end because a connection is available,
      # but another thread can snatch it up first. If timeout hasn't
      # passed but no connection is avail, looks like that happened --
      # loop and wait again, for the time remaining on our timeout. 
      before_wait = Time.now
      @queue.wait([@timeout - waited_time, 0].max)
      waited_time += (Time.now - before_wait)

      # Will go away in Rails 4, when we don't clean up
      # after leaked connections automatically anymore. Right now, clean
      # up after we've returned from a 'wait' if it looks like it's
      # needed, then loop and try again. 
      if active_connections.size >= @connections.size
        clear_stale_cached_connections!
      end
    end
  end
end

clear_reloadable_connections!() click to toggle source

Clears the cache which maps classes.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 146
def clear_reloadable_connections!
  synchronize do
    @reserved_connections = {}
    @connections.each do |conn|
      checkin conn
      conn.disconnect! if conn.requires_reloading?
    end
    @connections.delete_if do |conn|
      conn.requires_reloading?
    end
  end
end

clear_stale_cached_connections!() click to toggle source

Return any checked-out connections back to the pool by threads that are no longer alive.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 172
      def clear_stale_cached_connections!
        keys = @reserved_connections.keys - Thread.list.find_all { |t|
          t.alive?
        }.map { |thread| thread.object_id }
        keys.each do |key|
          conn = @reserved_connections[key]
          ActiveSupport::Deprecation.warn("Database connections will not be closed automatically, please close your
database connection at the end of the thread by calling `close` on your
connection.  For example: Keymap::Base.connection.close
") if conn.in_use?
          checkin conn
          @reserved_connections.delete(key)
        end
      end

connected?() click to toggle source

Returns true if a connection has already been opened.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 129
def connected?
  synchronize { @connections.any? }
end

connection() click to toggle source

Retrieve the connection associated with the current thread, or call checkout to obtain one if necessary.

connection can be called any number of times; the connection is held in a hash keyed by the thread id.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 93
def connection
  synchronize do
    @reserved_connections[current_connection_id] ||= checkout
  end
end

disconnect!() click to toggle source

Disconnects all connections in the pool, and clears the pool.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 134
def disconnect!
  synchronize do
    @reserved_connections = {}
    @connections.each do |conn|
      checkin conn
      conn.disconnect!
    end
    @connections = []
  end
end

release_connection(with_id = current_connection_id) click to toggle source

Signal that the thread is finished with the current connection. release_connection releases the connection-thread association and returns the connection to the pool.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 111
def release_connection(with_id = current_connection_id)
  conn = synchronize { @reserved_connections.delete(with_id) }
  checkin conn if conn
end

with_connection() { |connection| ... } click to toggle source

If a connection already exists yield it to the block. If no connection exists checkout a connection, yield it to the block, and checkin the connection when finished.

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 119
def with_connection
  fresh_connection = false
  connection_id = current_connection_id
  fresh_connection = true unless active_connection?
  yield connection
ensure
  release_connection(connection_id) if fresh_connection
end

Private Instance Methods

active_connections() click to toggle source

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 301
def active_connections
  @connections.find_all { |c| c.in_use? }
end

checkout_and_verify(c) click to toggle source

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 294
def checkout_and_verify(c)
  c.run_callbacks :checkout do
    c.verify!
  end
  c
end

checkout_new_connection() click to toggle source

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 285
def checkout_new_connection
  raise ConnectionNotEstablished unless @automatic_reconnect

  c = new_connection
  c.pool = self
  @connections << c
  c
end

new_connection() click to toggle source

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 277
def new_connection
  Keymap::Base.send(spec.adapter_method, spec.config)
end

release(conn) click to toggle source

# File lib/keymap/connection_adapters/abstract/connection_pool.rb, line 264
def release(conn)
  synchronize do
    if @reserved_connections[current_connection_id] == conn
      thread_id = current_connection_id
    else
      thread_id = @reserved_connections.keys.find { |k|
        @reserved_connections[k] == conn
      }
    end
    @reserved_connections.delete thread_id if thread_id
  end
end